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E coli Taxonomy Classification

Classification / August 12, 2017

Low-temperature electron micrograph of a cluster of E. coli bacteria, magnified 10, 000 times. Each individual bacterium is oblong shaped.

Escherichia coli (IPA: [ˌɛ.ʃəˈɹɪ.kjə ˈkʰoʊ.laɪ]), is one of the well known and significant species of bacteria living as gut fauna in the lower intestines of mammals. The number of individual E. coli bacteria in the feces that a human excretes in one day averages between 100 billion and 10 trillion. For this reason, E. coli has been used in water analysis as the indicator of fecal contamination. However, the bacteria are not confined to this environment, and specimens have also been located, for example, on the edge of hot springs.

E. coli is just one of many microbial species in mammals. For example, the large intestine of humans houses over 700 species of bacteria. These perform a variety of functions, with many benefits for the host organism. For example, bacteria produce enzymes capable of digesting many of the molecules that are indigestible to vertebrates, produce small amounts of vitamins for absorption into the blood, and help to prevent colonization by toxic bacteria. When located in the large intestine, E. coli also provides some of these same values for the host organism, assisting with waste processing, vitamin K production, and food absorption. Remarkably, the human immune system, which is so adept at recognizing invading organisms, does not mobilize against E. coli as a foreign object.

E. coli exists in ever increasing number of strains due to mutation and high frequency recombination (genetic transfer in conjugation). Though people generally understand E. coli as harmless intestinal flora, they are opportunistic and some of the strains have been identified as the serious causal agents of various illness. Furthermore, the health hazards associated with E. coli have become more complicated by the fact that some of the causal agents have developed resistance against usual antibiotics.

Since its discovery in 1885 by Theodor Escherich, a German pediatrician and bacteriologist (Feng et al. 2002), E. coli has been the subject of intense theoretical as well as practical research because of its abundance in close association with human beings. This has surely contributed to better understanding of natural phenomena, especially in the fields of genetics, genetic engineering, and microbiology.

General features

E. coli is a rod shaped, Gram-negative, facultative anaerobe, lactose-fermenting, non–endospore-forming microorganism. Its cell measures 1–2 µm in length and 0.1–0.5 µm in diameter. Its ten flagella are grouped in a peritrical arrangement. Strains of E. coli consist of colicinogeny factor or col plasmid (extra–chromosomal DNA ring), which is responsible for the production of bacteriocin called colicin, a toxin against other strains of the same genus.

Model of successive binary fission in E. coli

E. coli is a member of the family Enterobacteriaceae, which includes many genera, including well known pathogens such as Salmonella, Shigella, and Yersinia. One of the root words of the family's scientific name, "enteric, " refers to the intestine, and is often used synonymously with "fecal." All the different kinds of fecal coli bacteria (i.e., E. coli), and all the very similar (twin brother) bacteria that live in the soil or decaying plants (of which the most common is Enterobacter aerogenes), are grouped together under the name coliform. Technically, the "coliform group" is defined to be all the aerobic and facultative anaerobic, non–spore–forming, Gram–negative, rod–shaped bacteria that ferment lactose with the production of gas within 48 hours at 35°C (95°F). In the body, this gas is released as flatulence. Coliform is not a taxonomic classification but rather a working definition.

As with other bacteria, E. coli multiplies vegetatively by binary fission. Presence of F factor (fertility factor or sex factor) also has enabled the organism to undergo, here and there, conjugation for lateral DNA transfer between organisms. The F factor is a bacterial DNA sequence that allows a bacterium to produce a sex pilus and a conjugation bridge with another conjugant, necessary for conjugation. It contains 20 tra (for "transfer") genes and a number of other genetic sequences responsible for incompatibility, replication, and other functions. During conjugation in E. coli, genetic material thus may be transferred between the two conjugants and high frequency recombinants (strains with chromosomal DNA of another conjugant being received as an integral part of the plasmid, namely F–factor) may be produced. In addition, genetic transfer also may occur via transduction (transfer of gene via bacteriophage).

E. coli can survive outside of the host for a while, but the disinfection of all active bacteria can be easily carried out by pasteurization or simple boiling, with more rigorous sterilization process not required since the bacteria does not form spores.


A "strain" of E. coli is a group with some particular characteristics that make it distinguishable from other E. coli groups. These differences are often detectable only on the molecular level; however, they may cause changes in the physiology or life cycle of the bacterium, leading for example to the different level of pathogenicity (ability of an organism to cause disease in another organism).

Different strains of E. coli live in different kinds of animals, so it is possible to trace whether the fecal material in water came from from humans or from birds, for example.

New strains of E. coli arise all the time from the natural biological process of mutation, and some of those strains develop characteristics that can be harmful to their host animal. Although in most healthy adult humans such a strain would probably cause no more than a bout of diarrhea, and might produce no symptoms at all, in young children, people who are or have recently been sick, or in people taking certain medications, such a strain can cause serious illness and even death. The E. coli strain O157:H7 is one of hundreds of strains of the bacterium that is virulent and causes illness in humans (CCID/DBMD 2006).

Since E. coli and related bacteria possess the ability to transfer DNA via bacterial conjugation and bacteriophages, a new mutation can spread more widely through an existing population. It is believed that this process led to the spread of toxin synthesizing capability from Shigella to E. coli O157:H7.